Process for utilizing coal residues

ABSTRACT

COAL RESIDUES SUCH AS POWER STATION ASH OR MINING WASTE PRODUCTS, ARE MIXED WITH CALCIUM CARBONATE AND FIRED AT ABOUT 1300*C. THE FIRED MIXTURE IS THEN HEATED TO AT LEAST ABOUT 1500*C. TO TRANSFORM IT TO A MOLTEN SLAG. THE MOLTEN SLAG IS QUENCHED FROM ABOUT 1500*C. TO FORM GRANULES WHICH ARE COMMINUTED TO GET A PURE HYDRAULIC BINDER INTO WHICH IS INCORPORATED A SETTING AND HARDENING AGENT.

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United States Patent 3,759,730 PROCESS FOR UTILIZING COAL RESIDUES Leon-Jules Trief, 220 Rue des Baunes, Roclenge-sur-Geer, Belgium Filed Apr. 30, 1971, Ser. No. 138,873 Claims priority, applicati6o61 Luxembourg, May 4, 1970,

,850 Int. Cl. C04b 7/14, 7/24 US. Cl. 106-103 15 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process for utilising coal residues, particularly ash from .power stations and detritus from mining.

It is known that in power stations the combustion of coal for the production of electric power yields large quantities of ash. This ash has a physicochemical composition which is similar to that of natural pozzolana and can be incorporated into pulverised cement clinker for the production of special binders; hitherto this incorporation of ash has, however, had very limited application.

The object of the invention is to provide a process for utilising coal residues, in particular power station ash and detritus from mining, by transforming it in a simple and economic manner into a hydraulic binder capable of forming a real cement having the agglomeration properties of known cements, with at least equivalent hydraulic reactivity and initial and final setting properties, with a substantial resistance to decomposition after setting and with a structure similar to that of certain natural materials and providing an analogous hardness.

According to the invention there is provided a process for the production of hydraulic binder for use in the preparation of cement which comprises the following steps:

(a) Forming a mixture consisting of coal residues, advantageously power station ash or mining detritus, and calcium carbonate;

(b) Firing the mixture at a temperature of about 1300" C.;

(c) Heating the fired mixture to a temperature of at least about 1500 C. to transform it into a molten slag;

(d) Subjecting this molten slag to a rapid cooling in order to form granules; and

(e) Comminuting these granules to give as product a pure hydraulic binder. A setting and hardening agent may be incorporated in the product, or may be added at the time of use.

According to one embodiment of this process, fine or pulverised crude ash is added to the pure hydraulic binder in a quantity dependent on the desired final properties of mechanical resistance of the mixed binder thus obtained.

According to another embodiment the setting and hardening agent is a chemical base, preferably one characterised by the OH group, for example a strong base such as sodium hydroxide or potassium hydroxide. It can also be a chemical compound characterised by the 80;, group, for example calcium, potassium or sodium sulphate.

The setting agent has the elfect of developing at the time of hydration the setting and hardening properties of the hinder or, when used mixed with crude ash, of transforming the binder into a hydraulic binder whereof the ice physico-chemical cementing and pozzolanic characteristics are cumulative.

'In cases where the coal residue has a ferric oxide content exceeding 4%, the step of transforming the fired mixture into a molten slag is followed by a purification step which comprises heating the said molten slag to a temperature of up to 1800 C. so as to efiect separation of the metallic elements, the less dense slag floating on the separated denser material. After reducing the slag temperature to about 1500 C., the slag is rapidly cooled and transformed into granules which are then comminuted (steps (0) and (d) above).

The invention further provides an apparatus for continuously performing the process of the invention. This apparatus comprises:

(1) A furnace for firing and melting a mixture of coal residues and limestone to form a molten slag;

(2) An apparatus for rapidly cooling the molten slag with cold water to form granules; and

(3) Means for comminuting the granules to form a hydraulic binder. The apparatus may also include means for further treatment of the hydraulic binder so produced.

Other features of the invention will become apparent from the following description made with reference to the accompanying drawings, which serve to illustrate the invention without in any way limiting it.

FIG. 1 is a schematic view of an apparatus according to the invention;

FIG. 2 is a schematic view in longitudinal section through a complex firing and melting furnace.

FIG. 3 is a diagram giving the average weight composition of a portland cement, an aluminous cement and a binder according to the invention.

FIG. 4 is a Rankin diagram showing the composition of two binders according to the invention.

An ignited furnace 41 is charged with a separately prepared mixture consisting of about 40% by Weight of power station ash containing no unburned products and about 60% by weight of limestone with a grain size below 5 mm. This furnace comprises three separate but immediately successive sections. In the first section, 41a (FIG. 2), the charged materials are fired. This firing is performed at a temperature which varies according to the nature of the constituents of the mixture, but which should be below the temperature at which the materials begin to soften, this temperature being approximately 1300 C.

For firing the materials of the mixture coal, fuel oil or gaseous fuel may be used. The quantity of coal, where this is used, should be selected so that no unburned coal remains at the end of firing. Thus, with the above mixture, about 15% by weight of coal is used. On leaving the section 41a the fired mixture passes directly into section 4112, where it is completely melted. Section 41b is heated with a liquid or gaseous fuel or by electricity to a temperature of at least about 15 00 C.

As a result of the melting, a molten slag 57 is formed and flows continuously into a ladle 41c forming the third section of the furnace.

When the ash used has a ferric oxide content exceeding 4%, the temperature in the melting section 41b is raised to a maximum of about 1800 C. Thus, by exceeding the true melting temperature, separation of the dense metal elements is promoted. The metal elements are deposited at the base of the tank 58 and the molten slag 57 floats thereupon. The slag passes continuously into portion 41c, whilst the metal elements are dis-charged via a tap hole 410! and are collected in ingot moulds (not shown). Metal elements are normally present in power station ash and mining waste products in quantities varying between 5 and 30% by weight, depending on the source of the coal used. The metal elements recovered in ingot moulds could represent a valuable primary material for the metallurgical industry.

This separation step of the metal elements has the eifect of purifying and refining the slag, transforming it into a white binder having a commercial value much higher than that of conventional binders.

In the ladle 410 the temperature is lowered to about 1500" C., the liberated heat being recovered by any known means.

The molten slag leaving the ladle 41c flows continuously through a pipe 42 into which cold water is injected via a pipe 42a (FIG. 1), and the resulting molten granules fall into a drying drum 43. On leaving the drum 43 the moist granules are led directly to a grinder 44 of known type where they are ground in a wet medium, obviating the necessity of any prior drying of the granules and permitting the production of a very finely ground product. A grinding fineness corresponding to a specific area exceeding 5000 cm. g. measured by the Blaine method may thus be obtained. A pure hydraulic binder is obtained from the grinder 44 in the form of a relatively soft paste which can have various uses.

For example the paste can be directly introduced by a pipe 46 into a tanker vehicle 4611 and delivered to the user. In another case the paste can be pumped continuously out of the grinder 44 to a mixing tank 45 with which is associated a second tank 47 containing ash in the crude state. These two tanks discharge simultaneously onto two metering conveyers 45a and 47a leading to a mixer 51 which may discharge to various points, for example to a concrete mixer 54 for producing concrete, or to a storage silo 53 for placing in sacks, or to other points via a pipe 52. The pasty binder leaving the grinder 44 can also be led directly to the concrete mixer 54 or silo 53.

In still another case the paste leaving the grinder 44 is led to a mechanical drier 49 where part of its water is removed, the partly dried paste then being supplied to the customer. It can also be led to a complete drier 50 which removes all the moisture, resulting in a fine powder which can be supplied directly to the customer. Alternatively, this powder can be fed to a mixer 55 where it is mixed with dry crude ash from the tank 48, the resulting mixture passing into a storage tank 56.

A hardening agent can be added to the pure hydraulic binder obtained by the above-described process. The hardening agent may constitute a chemical base preferably one characterised by the OH group, for example a strong base such as sodium hydroxide or potassium hydroxide and is advantageously added in a quantity between 2 and of the weight of the binder. Sodium carbonate can also be added. Alternatively, the reagent may constitute a chemical compound characterised by the 80;; group e.g. sodium sulphate or calcium silicate, advantageously added in a quantity between 5 and of the weight of the binder. These chemical reagents can also be mixed with one another or with crude ash.

.The setting agent is added at the time of using the binder. If the pure hydraulic binder is transformed into dry powder the setting agent can be incorporated directly into this powder to form a complete binder similar to conventional binders.

A hydraulic binder obtained by the process of the invention permits a compression resistance of 250 kg./cm. to be obtained after 3 days and a minimum of 400 kg./ cm. after 28 days. The tests were performed on mortar bars by the international RILEM method.

It should be noted that the binder has a reduced thermal reaction as compared with a pure portland cement, so that contraction is reduced after hydration. Furthermore, after final hardening the hydrated binder is characterised by a substantially total resistance to decomposition.

Laboratory tests were performed on a binder according to the invention and various known binders, applying the RILEM international testing method, and the results obtained are given in the following table:

1 Belgian RILEM standard.

Binder A contained a total of 73% ash, 24% C210 and 3% NaOH.

The blast furnace cement tested contained 70% slag and 30% portland cement and the grinding fineness was 3700 cm. g.

Binder E contained a total of ash, 14% CaO and 3% NaOH.

For the pozzolanic binders the tests were performed on materials containing 60% portland cement and 40% pozzolana.

In the Rankin diagram of FIG. 4 are indicated a series of known products baed on CaO, SiO and A1 0 The references on this diagram correspond to the various products in the following list:

(1) F at lime (2) Poor lime (3) Light hydraulic lime (4) Slow natural cement (5) Super portland cement (6) H.R.I. portland cement (7) CPAL portland cement (8) Heavy hydraulic cement (9) Slagged lime (10) Grappiers cement (11) Mixed metallurgical cement (12) Blast furnace cement (l3) Supersulphated cement (14) CPA-LC. cement (l5) Gaize cement (16) Pozzolano-metallurgical cement (17) Italian pozzolana (18) Andernach trass (l9) Gaize (20) CPA portland cement (21) CPB portland cement (22) Iron cement (23) Slagged lime cement (24) Semi-slow setting natural cement (25) Quick-setting natural cement (26) Slag cement with clinker (27) Trass cement (28) CPA-C cement (29) Aluminous cement (30) Fly ash (31) Kilned slates and tiles (32) Gehlenite (33) Anorthite (a) Al O -3CaO or tricalcium aluminate (b) 3Al O -5CaO or pentacalcium aluminate (c) AI O 'CaO or monocalciurn aluminate (d) 5Al O -3CaO or tricalcium pentaaluminate (e) SiO -3CaO or tricalcium silicate (f) 'SiO -2CaO or dicalcium silicate Two hydraulic binders according to the invention are located respectively at points 34 and 35 of this diagram, the first containing about 28% CaO, about 42% SiO and about 30% A1 0 and the second about 20% CaO, about 50% SiO and about 30% A1 0 FIG. 3 gives the weight percentages of a portland cement (A), an aluminous cement (B) and two binders according to the invention (C and D).

The above-described process can also be applied to the production of binders from the waste products of coal mines. The latter generally consist of heaps of excavated material from below ground, consisting mainly of silicoalumino-limestone materials, metallic materials and more or less adulterated coals.

According to the process of the invention the coal contained in the mining waste products is profitably used for firing other petrified materials forming the same. In this case the mining waste products, i.e. the coal, metallic elements and the C210, Si and A1 0 constituents are analysed and it is thus possible to know whether the quantity of coal will suffice to fire the calcarous, silicous and aluminous materials contained therein. Thus, any possible corrections which are necessary can be made for forming the melting and firing a furnace charge and applying the process of the invention. i

What is claimed is:

1. A process for utilizing coal residues selected from the group consisting of power station ash and mining waste products, comprising forming a mixture of said coal residues and calcium carbonate, firing this mixture at a temperature of about 1300 C., thereafter heating the fired mixture to a temperature of at least about 1500 C. to form a molten slag, quenching said slag with water to form wet granules, wet grinding said granules to obtain a hydraulic binder, and finally admixing a setting and hardening agent with said hydraulic binder.

2. A process according to claim 1 characterized in that said coal residues have a ferric oxide content greater than 4%, heating the molten slag to a temperature up to 1800 C. so as to separate iron from the molten slag, and reducing the temperature of the molten slag to about 1500" C. prior to said quenching.

3. A process according to claim 1 characterized in that crude ash is added to the hydraulic binder in a quantity to enhance mechanical resistance.

4. A process according to claim 1 characterized in that a mixture containing about calcium carbonate and about 40% coal residue is treated.

5. A process according to claim 1 characterized in that the setting and hardening agent is a chemical base.

6. A process according to claim 5 characterized in that the setting and hardening agent is a strong base characterized by the OH group.

7. A process according to claim 6 characterized in that the setting and hardening agent is sodium hydroxide.

8. A process according to claim 6 characterized in that the setting and hardening agent is potassium hydroxide.

9. A process according to claim 5 characterized in that the setting and hardening agent is sodium carbonate.

10. A process according to claim 1 characterized in that the setting and hardening agent is a compound characterized by the S0 group.

11. A process according to claim 10 characterized in that the setting and hardening agent is sodium sulphate.

12. A process according to claim 10 characterized in that the setting and hardening agent is calcium sulphate.

13. A process according to claim 10 characterized in that the setting and hardening agent is potassium sulphate.

14. A process according to claim 1 characterized in that the setting and hardening agent is a strong base added in a quantity of 2-5 by weight of the hydraulic binder.

15. A process according to claim 1 characterized in that the setting and hardening agent is a compound characterized by the group added in a quantity of 5-10% by weight of the hydraulic binder.

JAMES E. POER, Primary Examiner 

